The present invention relates generally to current mirror amplifiers and, more specifically, to a current mirror amplifier designed for high temperature environments.
Most IC component characteristics are temperature dependent, however, only a few of them are of particular importance for high temperature circuit design. Leakage current is probably the most obvious problem parameter. In IC junctions, it doubles approximately every 11.degree. C. reaching a few microamperes at 300.degree. C. Leakage of this magnitude swamps the design currents used in low current strings of many operational amplifiers and other linear circuits. It also significantly increases the static power dissipation of CMOS circuits. Its voltage dependence leads to a leakage resistance which may significantly load the high impedance modes employed in some analog circuitry.
H.sub.FE increases with temperature which is beneficial to most circuit designs. However, between 200.degree. C. and 300.degree. C., I.sub.CBO becomes large enough that base current changes sign in most analog IC transistors. This defeats the operation of several commonly employed circuit configurations, such as simple current mirrors composed of a diode connected transistor whose base is connected to the base of the mirror transistor. Incremental current gain remains high even when base current is reversed since it is independent of I.sub.CBO.
The resistivity of doped semiconductor layers increases until the intrinsic carrier density, n.sub.i, which has exponential temperature dependence, is approximately equal to the doping impurity concentration N. Above the temperature, called the intrinsic temperature, at which this occurs, resistivity begins to decline rapidly due to the increased total carrier concentration N+n.sub.i. The smaller N, the lower the intrinsic temperature.
For 4 ohm-cm material which is commonly used for linear NPN collectors and CMOS substrates, the maximum resistivity of 12 ohm-cm occurs at about 200.degree. C. and n.sub.i =N at about 325.degree. C. where resistivity is back down to about 4 ohm-cm. When both sides of a PN junction reach their intrinsic temperature, the junction loses its rectifying characteristic. This is one ultimate limit to high temperature circuit operation.
Many circuits have operating currents which are proportional to diffused resistors. At high temperature, their current may change by as much as a factor of two due to the resistor IC. Changes of that magnitude can induce large shifts in performance, in some cases even causing circuits to lose functionality.
The transconductance of a bipolar transistor given by the expression g.sub.m =(QI.sub.E /KT) is inversely proportional to absolute temperature. It decreases by about a factor of two over the temperature range 25.degree. C. to 300.degree. C. having adverse effect on a number of linear circuit parameters including voltage gain.
The V.sub.F of PN diodes and V.sub.BE of bipolar transistors have a negative temperature coefficient of about 2 mV per .degree.C. At 300.degree. C. V.sub.BE of a typical IC transistor is about 100 mV. Many circuit bias schemes which depend upon V.sub.BE fail at high temperature often by forcing devices into saturation. Current matches set by matched V.sub.BE's are more easily disrupted when the V.sub.BE's are this low. Noise margins in the several types of logic where the margin is proportional to V.sub.BE become very small at 300.degree. C.
Since current mirrors are based on the principle that the V.sub.BE of the input leg is used as a control of the V.sub.BE of the output leg, current mirrors are very susceptible to disruption in the high temperature environment. Also, the input transistor will go into saturation destroying the effectability of the input stage. Thus, there exists the need for a current mirror design which is capable of operating in the high temperature environments.